The energy obtained from biomass is highly positive from the point of view of energy. For instance, the energetic efficiency of the so-called short-rotation biomass is 89.5%. and the rate of liquid energy is 9.48 times higher. However, in spite of this fantastic energetic efficiency, biomass cannot compete with fossil fuels due to the high costs resulting from the large number of steps required to produce it and also due to the difficulty in handling the raw biomass, which renders it not very practical.
The following points related to the process for producing biomass should be taken into account: 1) planting and cultivation (propagation); 2) expenses with nutrients (fertilization); 3) exposure to the sun; 4) temperature; 5) precipitation; 6) conditions of soil and water, 7) harvesting method; 8) resistance to diseases; 9) competition in the area with production of foodstuffs, pastures and fibers; 10) area availability; 11) transport of the raw biomass.
Biomasses are composed of cellulose, hemicellulose and lignin, the composition position being exemplified in Table 1, and microstructure according to FIG. 1.
TABLE 1Typical Composition - Pine Tree (%)Cellular wallhemicellulosecelluloseligninLM - middle lamella——3.0P - primary wall1.40.78.4S - secondary wallS13.76.1310.5S218.42.79.1S35.20.8Total28.740.331.8
The cellular walls are composed of macrofibrillae, microfibillae, micellae and cellulose molecules. The nuclei of the cells (cytoplasm) is composed of aqueous solutions. The following formulas represent the approximate estimates of the specific surface (area per unit of mass) of the biomass in the hypothesis of its microstructure being completely released.
1—Geometry with a Square section and length l (S and M: cell surface and cell mass).             S      =              4        ⁢                                   ⁢        b        ⁢                                   ⁢        l              ;          M      =                                    4            ⁢                                                   ⁢            b            ⁢                                                   ⁢            l            ⁢                                                   ⁢            e            ⁢                                                   ⁢            ρ                    ∴                      S            M                          =                                            4              ⁢                                                           ⁢              b              ⁢                                                           ⁢              l                                      4              ⁢                                                           ⁢              b              ⁢                                                           ⁢              l              ⁢                                                           ⁢              e              ⁢                                                           ⁢              ρ                                =                      1                          e              ⁢                                                           ⁢              ρ                                                                        broadness          ⁢                                           ⁢          of          ⁢                                           ⁢          the          ⁢                                           ⁢          cell                                      b          =                      10            ⁢                                                   ⁢            µm                                                        thickness          ⁢                                           ⁢          of          ⁢                                           ⁢          the          ⁢                                           ⁢          cell          ⁢                                           ⁢                      wall            :                                                e          =                      1.0            ⁢                                                   ⁢            µm                                                                                                                                         ρ                =                                  1.5                  ⁢                                                                           ⁢                  g                  ⁢                                      /                                    ⁢                                      cm                    3                                                                                                                          =                                  1.5                  ×                                      10                    6                                    ⁢                                                                           ⁢                  g                  ⁢                                      /                                    ⁢                                      m                    3                                                                                          
2. Specific area of the macrofibrillae, microfibrilla, miscellae and cellulose molecules.       S    =          π      ⁢                           ⁢      ϕ      ⁢                           ⁢      l        ;      M    =                                                      π              ⁢                                                           ⁢                              ϕ                2                                      4                    ⁢          l          ⁢                                           ⁢          ρ                ∴                  S          M                    =                                    π            ⁢                                                   ⁢            ϕ            ⁢                                                   ⁢            l                                                              π                ⁢                                                                   ⁢                                  ϕ                  2                                            4                        ⁢            l            ⁢                                                   ⁢            ρ                          =                  4                      ϕ            ⁢                                                   ⁢            ρ                              
2.a—Specific area of the macrofibrilla (φ=50 nm; Macropores>50 nm)       S    M    =            4              50        ×                  10                      -            9                          ×        1.5        ×                  10          6                      =          53      ⁢                           ⁢              m        2            ⁢              /            ⁢      g      
2.b—Specific area of the microfibrillae (φ=50/4=12.5 nm; Mesopores 2 nm<φ<50 nm)       S    M    =            4              12.5        ×                  10                      -            9                          ×        1.5        ×                  10                      -            6                                =          213      ⁢                           ⁢              m        2            ⁢              /            ⁢      g      
2.c—Specific area of the miscella (φ=(12.5/4)nm=3.1 nm; Micropores φ<2.0 nm)       S    M    =            4              3.7        ×                  10                      -            9                          ×        1.5        ×                  10          6                      =          860      ⁢                           ⁢              m        2            ⁢              /            ⁢      g      
2.d—Specific area of the molecules of cellulose (3.1/6)nm=0.517 nm)             S      M        =                  1                  0.517          ×                      10                          -              9                                ×          1.5          ×                      10            6                              =              1290        ⁢                                   ⁢                  m          2                ⁢                  /                ⁢        g                                                      N            =                        ⁢                                          1                +                                  6                  ⁢                                                            ∑                      0                      n                                        ⁢                                          n                      ⁢                                                                                           ⁢                      i                                                                                  =              1                                ,                      (                                          1                +                6                            =              7                        )                    ,                      (                                          1                +                6                +                12                            =              19                        )                    ,                                                        ⁢                                    (                                                1                  +                  6                  +                  12                  +                  18                                =                37                            )                        .                              
The theoretical specific area for the cell is of about 0.7 m2/g, of about 50 m2/g for the macrofibrillae, of about 200 m2/g for the microfibrillae, of about 900 m2/g for the miscealla, and of about 1300 m2/g for the molecules.
As far as solid fuels are concerned, their conventional combustion comprises 5 zones: first non-reactive solid zone (heating and drying), second reaction zone of condensed phase (solid pyrolysis), third reaction zone of gaseous phase (pyrolysis of gaseous phase and oxidation), fourth primary combustion zone (gaseous phase), fifth post-flame reaction zone (secondary combustion). The specific kinetics and reactions of each zone is not completely known yet.
FIG. 2 illustrates the conceptual model of conventional combustion for wood. Wood is anisotropic and hygroscopic, and its fibers (tracheids) are hollow and have a length of from 3.5 to 7.0 mm in soft wood, and from 1 to 2 mm in hard wood. The linked water is of about 23%, and the total moisture reaches 75%. Cellulose, hemicellulose and lignin behave as polyalcohols wherein the main functional group is the OH group. Cellulose is a linear polysaccharide of anhydrous glucose with 1→4-β glucoside bonds. After oxidation, the functional groups are carbonylic, ketone and carboxylic groups. On the other hand, hemicellulose is a polysaccharide with branched chain, the main components of which are 4-O-methylglucoroxylanes in hard wood and glucomanes in soft wood. The main functional groups thereof are carboxylic, methylic and hydroxylic groups. Lignin, on the other hand, is a tridimensional backbone of 4 or more substituted phenylpropane units. The basic constitutive blocks are guayaquil alcohols (soft wood) and seringyl alcohol (for the two types of wood), and the dominant bonds are β-O-4.
The structures of cellulose and lignin are highly oxygenated and the location of the functional groups is useful in understanding the mechanisms of pyrolysis and oxidation.
For the purpose of comparison, it is observed that the structure of the mineral coal is aromatic, it has few hydroxylic functional groups and β-O-4 bonds. Nitrogen and sulfur are part of the structural rings with little nitrogen existing in the amine form. The fact that the oxygen content is very low in coals when compared with wood is highly significant, since it imparts greater reactivity to the latter.
In the conventional combustion of wood the drying stage involves, in fact, 4 steps, namely 1) energy required for heating the wood up to 100° C. (373° K)=0.08×100×(1−TU) kJ/kg, wherein TU is the moisture content (percentage); 2) energy required for heating water=4.2×100 kJ/kg; 3) energy required for vaporizing the water=2.26 MJ/kg; and 4) energy required for releasing the linked water 15.5×TU kJ/kg (average). The predominant value is the energy from vaporization of water.
The heating stage comprises three factors that have significant influence: the first one is the energy for heating up to the pyrolysis temperature (500-625° K); the wood specific heat is 1113 J/g at 273° K and 1598 J/g at 373° K, while the specific heat of the wood with 35% of moisture is 2.343 J/g at 300° K. Secondly, there is the influence of the moisture preventing the particle core be heated up to the temperature at which water is evaporated and establishing the reaction states. The third factor of influence is the moisture in the increase of the thermal conductivity of the wood particle, which may at most double its value. In addition to its influence on the drying and heating, moisture also causes significant effects on the solid state pyrolysis.
The next stage is the solid pyrolysis step. In this combustion zone, reactions of cleavage of the molecules into gaseous fragment and condensation reactions prevail, whereby coal is produced (tar resulting into 3 final fractions: a gaseous one, a liquid one, and a solid one—coal). The pyrolysis temperatures are: hemicellulose (500-600° K), cellulose (600-650° K) and lignin (500-773° K). Table 2 show the pyrolysis products from cellulose and xylan, with a high tar content that causes a secondary combustion close to the oils for the wood.
TABLE 2Pyrolysis Products from Cellulose (873° K) and Xylan (773° K)ProductCellulose (% P)Xyilan (% P)Acetaldehyde1.52.4Acetone Propinaldehyde0.00.3Furanics0.7TrPropenol0.80.0Methanol1.11.32-MethylfuranTr0.02,3-Butanedione2.0Tr1-Hydroxy-2-Propan glycoxal2.80.4Acetic acid1.01.52-Furaldehyde1.34.55-Methyl-2-Furaldehyde0.50.0CO26.08.0H2O11.07.0Coal5.010.0Tar66.064.0Tr = trace 
The opening of aromatic rings is an intermediate step in forming the volatile material, generating acetic acid and acetaldehyde, which are decomposed by decarboxylation of acetic acid (CH3COOH→CH4+CO2) and decarbonilation of the acetaldehyde (CH3CHO→CH4+CO). From the hemicellulose, the resulting product is C2H4 and CO from the propanol. In the next zones, there will be sequence in the pyrolysis and oxidation, giving CH4, C2H4, CO and CO2 as final products.
The pyrolysis of lignin is different in comparison with the hemicellulose and cellulose and at 823 K it produces the following components: coal (55%), gaseous fraction (45%) composed of CO (50%) CH4 (38%), CO2) 10%) and C2H6 (2%). The tar is composed of phenylacethylene, antracene and naphthalene. Table 3 shows the formation of coal in the pyrolysis of several different materials.
TABLE 3Coal Formation in the Pyrolysis of Several Different Materials (673 K)MaterialCoal (% P)Cellulose14.9Poplar (wood)21.7Larch (wood)26.7Aspen (branches)37.8Douglas (bark)47.1Klason Lignin59.0
Moisture also has a considerable influence on the particle pyrolysis since it causes an enormous difference in temperature between the particle core and the periphery thereof (400° K), creating a physical separation between the heating and drying zone and the pyrolysis zone. The dominant influence of moisture is to reduce the flame temperature of the burner, directing the product to coal formation and reducing the rate of pyrolysis. The theoretical flame temperature of the wood combustion is given by:Ta=1920−(1.51[TU/(1−TU)]×100)−5.15 XexAr
wherein Ta (K) is the adiabatic flame temperature, TU is the fraction of the moisture contents, and X exAr is the percentage of air excess. In addition to the reduction of the adiabatic temperature, there is an increase in the air excess, given by:XexAr(%)=40[TU/(1−TU)]
For TU>33%, Ta=1740° K and for TU=50%, Ta=1560° K and consequently there is a decrease in the volatile content and an increase in the coal content. Finally, one should cite that the ashes reduce the local temperature and catalyze the formation of coal.
Next, the pre-combustion reaction occurs, which represent the cleavage of volatile material into fragments of radicals dominated by reactions of initiation of chains of the type:R−R→R+R′(368 kJ/mol)R″−H→R″+H(410 kJ/mol)wherein R=C2H6, CH3, etc. e R″=methylic group.
In wood, the first reaction is most probable due to its lower energy, and an example thereof is given below:C2H6+M→2CH3+M2CH3+2C2H6→2CH4+2C2H5M+C2H5→H+C2H6+MH+C2H6→H2+C2H5
wherein M is a heat (ash or vapor)-removing particle or molecule. If R″ contains two or more carbon atoms, the C—C bond is broken preferably instead of the C—H bond. In addition to the reactions of chain initiation, the pre-combustion zone includes reduction reactions with recombinations of radicals R+R′→R−R′, especially if the pre-combustion zone is spatially broad. An example thereof is the recombination of nitrogen forming N2 instead of NOx.
After the pre-combustion reactions, primary combustion reactions occur oxygen and fuel mixed in the primary combustion zone results in a number of reactions of free radical, producing CO2 and H2O.RH+O2→R+HOOCH3+O2+M→CH3O2+MCH3O2→CH2O+OH
HCO and CO (CH2O+(1/2)O2→HCO+OH or CH2O+O2→CO+2HO) are formed from CH2O, and their concentration is maximized at flame temperatures of 1320 K, which is the wood combustion temperature.
Finally, the post-combustion reactions occur: the processes of wood combustion occur at low temperature, and reactions of chain end occur in the secondary combustion. The hydroxyl radical (CH2O) is of great significance when it is present at high concentrations. The main end reactions are:HCO+OH→CO+H2OCO+OH→CO2+HCO+O2→CO2+O
the latter being of lesser importance in this zone. The CO2 production from CO is controlled by the OH concentration, which is relatively high for low temperature systems (wood). It follows that the chain end is the recombination of H and OH groups aided by heat-removing species (M). The C:H ratio is relatively high for soft wood (1:1.45) and hard wood (1:1.37) compared with mineral coals (1:017). The wood solid pyrolysis produces water, CH4, C2H4, and C2H6, resulting in a substantial amount of hydrogen in the volatile gases to increase the concentration of hydroxyl radical for a complete and rapid oxidation (greater reactivity). There is no complete expressions in the literature for this system, due to the large number of variables associated to the oxidation of the wood volatiles.
In the combustion of (wood) charcoal, the charcoal obtained from the pyrolysis is porous and contains various free radicals for O2 attack. In addition, it contains oxygen and hydrogen, its empirical chemical formula being C6.7H,3.3O. Three mechanisms were proposed for the charcoal oxidation, it being recognized that the combustion rate is limited by the sites of free radicals on its surface. The charcoal oxidation is also limited by the mass transport. The first mechanism is the Boudouard, as the general indicator of charcoal combustion.C+O2→2CO
This reaction is highly endothemic with the following reaction constants: 1.1×10−2 (800° K) and 57.1 (1200°). The CO released is volatile and its combustion is completed in the flame out of the particle. The second mechanism is the chemical adsorption of O2 directly on the coal. The activation energy of the O2adsorption on the porous surface of the coal ranges from 54 kJ/mole to 10 105 kJ/mol, respectively, for chemically adsorbed quantities from zero to 2.5 moles of O2 per gram of coal. The chemical adsorption reactions are:C*+O2→C(O)*→C(O)m→CO+CO2C*+O2→COes→CO+CO2
The asterisk indicates an active site of reaction, m stands for moveable species, and es stands for stable species. The charcoal active sites can be generated by the mechanism of pyrolysis. The third mechanism of charcoal oxidation involves reactions of hydroxyl radicals in the active sites given by:2OH+C→CO+H2OOH+CO→CO+H
Hydroxyl radicals are internally generated by homolytic cleavage of the various hydroxylic functional groups existing in the wood or dissociation of the moisture released by the fuel. The moisture influence on the coal oxidation are not well known, as in the case of the pyrolysis of wood. It is speculated that the moisture “deletes” the sites, reducing the rate of coal oxidation. The presence of moisture delays the rate of oxidation of charcoal.
In short, the wood combustion is a multistage process that involves heating and drying, solid state pyrolysis, producing volatile compounds and coal, reactions of gaseous phases (pre-combustion, primary combustion and post-combustion) and combustion of the coal. The various functional groups existing in wood generate a significant number of volatile products from the solid pyrolysis of particles, the various functional groups and the high aliphatic contents increasing the reactivity of wood, contributing to the high proportion of flames in the combustion of the wood with respect to mineral coal. The moisture increases the thermal conductivity, results in greater production of coal in the solid state pyrolysis, increases the concentration of hydroxyl groups for the reactions of gaseous phase and of the coal, and reduces the oxidation rate of the coal, decreasing its temperature and “deleting” the reactive sites.
In view of the complexity and the operational disadvantages presented by the conventional combustion processes, it was desirable to develop a new fuel from biomass that could meet the essential requirements of combustion and overcome the technical drawbacks of the known fuels.
In this regard, various studies have been carried out for the development of new fuels from biomass and some attempts have already presented satisfactory results, as in the case of a cellulignin fuel mentioned in the article “Cellulignin: a new thermoelectric fuel” by Datro G. Pinatti, Christian A. Vieira, Jose A. da Cruz and Rosa A. Conte, which relates to a product from generic cellulignin obtained by a process of pre-hydrolysis of biomass without optimized control. However, it was still desired to obtain a fuel that would present even more advantageous results, mainly from the economic point of view and the applications thereof in the main thermoelectric technologies: ovens, boilers, gas turbines and generation of energy by hydrodynamic magnet (MHD).
Therefore, the objective of the present invention it to provide a new cellulignin fuel with catalytic properties that will meet these market requirements with improved combustion characteristics.